yield question with append

[gnomegordon]

when I try to use print the output the yield result, everything is shown as expected. The code and output is as below:

def triangles():
    oo = [1] 
    yield oo 
    oo.append(1)
    yield oo  
    oo.append(1)
    yield oo 
    oo.append(1)
    yield oo 

# Collect the first 3 rows of Triangle
n = 0
for t in triangles():
    print (t)
    n += 1
    if n == 3:  
        break

the output is:

Output:
[1]
[1, 1]
[1, 1, 1]

However when I tried to append the yield result further, like below

n = 0
results = []
for t in triangles():
    ## print (t)
    results.append(t)
    n += 1
    if n == 3:  
        break

# Print the results
for t in results:
    print(t)

the output becomes:

Output:
[1, 1, 1]
[1, 1, 1]
[1, 1, 1]

Actually it is hard for me to understand the result. why the 1st and 2nd row changes after further appending?